Forcing fat and water to stay mixed by breaking one into droplets small enough that a third substance can hold them apart.
Technique at a glance
A compact visual model of the lesson. Use the full mechanism and success criteria below when you practice.
Concept
Forcing fat and water to stay mixed by breaking one into droplets small enough that a third substance can hold them apart.
What changes
Fat and water do not dissolve in each other, so left alone they separate into two layers — the arrangement with the least contact between them.
What good looks like
The mixture turns opaque. Two separate layers are translucent; an emulsion scatters light and goes pale and matte.
Practice counts when you cook
Reading this brief does not change your School state. Finish a real practice dish in Cook mode and log the cook; that cook is the evidence that advances the skill.
What to focus on
Mechanism
Fat and water do not dissolve in each other, so left alone they separate into two layers — the arrangement with the least contact between them. An emulsion suspends one inside the other as droplets, and it needs two things at once: mechanical energy to tear the droplets small, and an emulsifier to coat each droplet so it cannot merge back. Emulsifiers are molecules with a water-loving end and a fat-loving end — lecithin from egg yolk, casein from butter and cream, the mucilage in mustard, dissolved starch from pasta water. Whisking without an emulsifier gives you a mixture that separates within a minute. Adding fat faster than the whisk can break it leaves droplets uncoated, they coalesce, and the sauce splits.
Success criteria
The mixture turns opaque. Two separate layers are translucent; an emulsion scatters light and goes pale and matte.
It thickens noticeably even though nothing was added that thickens — the resistance comes from packed droplets.
It coats the whisk or the back of a spoon and holds a line drawn through it.
The sound changes as it forms, from a thin slosh to a heavier, quieter sound.
Failure guards
The sauce separates into an oily layer and a watery one.
Fat added faster than it could be broken up, or not enough emulsifier for the amount of fat. Roughly 60 g of butter per egg yolk is the working ceiling for hollandaise; past it there is nothing left to coat new droplets.
It formed, then broke a few minutes later.
Temperature. Heat thins the fat and speeds droplets into each other; in a yolk-based sauce, heat also cooks the emulsifier out of existence.
It never thickened at all and stayed thin and clear.
No agitation, or no emulsifier. A vinaigrette stirred gently and without mustard is just oil sitting on vinegar.